1. Field of the Invention
This invention relates to a directional filter used in data handling systems and, more particularly, to a variable bandwidth microwave directional filter which is dielectrically tuned to resonant frequency and additionally attenuated for broadened bandpass filter bandwidth and reduction of coupling insertion loss.
2. Description of the Prior Art
In data handling systems requiring the implementation of microwave technology for broad bandwidth, high frequencies, and system configuration needs, data buses are used to serve the systems and to control data transfer. Buses can be utilized for unidirectional or bi-directional transmission and can be adapted to the frequency bandwidths requirements of the system.
Coupling of the data bus to transmission lines to perform the required functions can be accomplished by a directional coupler with a series bandpass filter. Conventional microwave cavities are used as filtering elements where nondirectional coupling is applied and standing waves are involved. When directional coupling is applied, waves progressing in one direction are obtained. Directional filtering of traveling waves is achieved by coupling a ring-shaped transmission line to the two transmission lines. The ring is closed and is any integral number of wavelengths in circumference. One approach of this type directional filter emphasizes a limited strip line or microstrip application as discussed in two papers, one by Cohn, S. B. and Coale, F. S., "Directional Channel-Separation Filters," Nat. Conv. Record IRE, 1956, Part 5, p. 106 and Proc. IRE 1956, 44, p. 108, and the other by Coale, F. S., "A Travelling-Wave Directional Filter," Trans. IRE, 1956, MTT-4, p. 256.
The properties of microwave circuits utilizing a waveguide coupled to a single transmission line is discussed by F. J. Tischer in his paper "Resonance Properties of Ring Circuits," Trans. IRE, MTT- 5, 1957, p. 51. The coupling of the ring circuit to the transmission line is through two quarter wave spaced apertures in the waveguide. The wavelengths at which resonances occur in such a ring guide is expressed by ##EQU1## where: N is the integral number of wavelengths in the ring, L is the mean circumference of the ring, and A is the width of the guide in the ring where the ring guide is a rectangular waveguide. Adjustment of the guide width, A, by screwing one half of the guide in or out relative to the other results in a variation of the wavelength in the guide and of the resonant wavelength and is thereby a means of tuning the ring circuit. Problems arise with mounting ring circuits of this type since the adjustment results in a movement in the relative positions of the waveguide connection flanges. The bandwidth of such a resonating device is a function of the quality factor, Q, of the ring resonator. A decrease in Q results in increased resonator bandwidth and an increase in Q results in decreased bandwidth. Q is dependant upon such factors as the characteristics of the ring transmission line, the number of wavelengths in the ring and any additional attenuation introduced in the ring. For example, a decrease in the height of the ring reduces Q just as an addition of attenuation decreases Q. The lowest value of Q achieved by Tischer is of the order of 2700 which would seem to indicate a small chance of large instantaneous bandwidth, which would tend to limit the application of the device for data buses.
Subsequent designs extend Tischer's concept by adding an output guide to achieve a directional filter status of the device. U-shaped input and output guides coupled to a ring structure are used whereby tuning is achieved by mechanically adjusting the width of the ring guide. This mechanical adjustment creates the same problem as Tischer's device in that the relative positions of the waveguide connection flanges are varied resulting in mounting difficulties.
Further development of the ring filter concept is discussed in a paper by Ohtomo, I., and Shimada, S., "A Channel-Dropping Filter Using Ring Resonator for Millimeter-Wave Communication System," Elect. Comm. Japan, Vol. 52--B, No. 5, 1969, p. 57. U-shaped input and output guides are used which are coupled to a single or double ring resonator. Coupling of the rings to the guides is achieved through the side walls of the ring as well as the top walls. Where the coupling is through the top wall the variation of the guide width to tune the resonator cannot be used. In its place, a dielectric rod is gradually introduced to tune the ring resonator. A problem which arises with the device described by Ohtomo is that the broadband match of the coupled circuits depends upon the amount of the insertion of the dielectric rod into the ring cavity. The dielectric rod, if not tapered or gradual, results in a mismatch of the impedances between the ring resonator and transmission lines. Another problem arising in the utilization of this device in data bus systems is the more difficult mounting of the U-shaped guides. Space and economical factors are considered in the design of data buses for microwave systems applications and the U-shaped waveguide structure is a less suitable mounting arrangement.
Most, if not all, directional filters of the prior art have commonality in that they couple all of the signal at the resonant frequency of the ring from the input to the output line. Insertion loss due to tight coupling with little or no ring attenuation is excessive in the frequency band of the directional filter. Since the insertion loss is high, none of the signal is retained in the input line when coupled to the output line, all of the signal being transferred. Therefore, other couplers in series would be starved for signal thereby not permitting any additional coupling of other lines within this frequency bandwidth. Furthermore, in certain prior art arrangements, the means for varying the filter bandwidth by changing the attenuation or the quality factor, Q, is directly related to the tuning means thereby minimizing the degrees of freedom for attaining optimum results.